In general, a liquid developer for electrophotography is prepared by dispersing an inorganic or organic pigment or dye such as carbon black, nigrosine, phthalocyanine blue, etc., a natural or synthetic resin such as an alkyd resin, an acrylic resin, rosine, synthetic rubber, etc., in a liquid having a high electric insulating property and a low dielectric constant, such as a petroleum aliphatic hydrocarbon, etc., and further adding a polarity-controlling agent such as a metal soap, lecithin, linseed oil, a higher fatty acid, a vinyl pyrrolidone-containing polymer, etc., to the resulting dispersion.
In such a developer, the resin is dispersed in the form of insoluble latex grains having a grain size of from several nm to several hundred nm. In a conventional liquid developer, however, a soluble dispersion-stabilizing resin added to the liquid developer and the polarity-controlling agent are insufficiently bonded to the insoluble latex grains, thereby the soluble dispersion-stabilizing resin and the polarity-controlling agent are in a state of easily dispersing in the liquid carrier. Accordingly, there is a fault that when the liquid developer is stored for a long period of time or repeatedly used, the dispersion-stabilizing resin is split off from the insoluble latex grains, thereby the latex grains are precipitated, aggregated, and accumulated to make the polarity thereof indistinct. Also, since the latex grains once aggregated or accumulated are reluctant to re-disperse, the latex grains remain everywhere in the developing machine attached thereto, which results in causing stains of images formed and malfunctions of the developing machine, such as clogging of a liquid feed pump, etc.
For overcoming such defects, a means of chemically bonding the soluble dispersion-stabilizing resin and the insoluble latex grains is disclosed in U.S. Pat. No. 3,990,980. However, the liquid developer disclosed therein is still insufficient although the dispersion stability of the grains to the spontaneous precipitation may be improved to some extent. Also, when the liquid developer is actually used in a developing apparatus, the toner adhered to parts of the developing apparatus solidified to form a film and the toner grains thus solidified are reluctant to redisperse and are insufficient in re-dispersion stability for practical use, which causes the malfunction of the apparatus and staining of duplicated images.
In the method of producing resin grains described in aforesaid U.S. Pat. No. 3,990,980, there is a very severe restriction in the combination of a dispersion stabilizer to be used and monomer(s) being insolubilized for producing mono-dispersed latex grains having a narrow grain size distribution. Mostly, the resin grains produced by the aforesaid method are grains of a broad grain size distribution containing a large amount of coarse grains or poly-dispersed grains having two or more different mean grain sizes. In the aforesaid method, it is difficult to obtain mono-dispersed resin grains having a narrow grain size distribution and having a desired grain size, and the method often results in forming large grains having a mean grain size of 1 .mu.m or larger or very fine grains having a mean grain size of 0.1 .mu.m or smaller. Furthermore, there is also a problem that the dispersion stabilizer used must be prepared by an extremely complicated process requiring a long reaction time.
Furthermore, for overcoming the aforesaid defects, a method of improving the dispersibility, redispersibility and storage stability of resin grains by forming insoluble dispersed resin grains by polymerizing a monomer being insolubilized in the presence of a polymer utilizing a di-functional monomer or a polymer utilizing a macromolecular reaction is disclosed in JP-A-60-185962 and JP-A-61-43757 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
On the other hand, a noticiable progress has recently been made in shortening the operation time in an electrophotomechanical system and an improvement of quickening a development-fix steps in the system has been made.
Also, the rationalization of an electrophotomechanical system has been greatly required and practically, it has been attempted to prolong the maintenance time of a printing plate making machine. In this attempt, a liquid developer which can be used for a long period of time without being renewed has been required.
The dispersed resin grains produced by the methods disclosed in aforesaid JP-A-60-185962 and JP-A-61-43757 yet show an unsatisfactory performance with respect to the dispersibility and re-dispersibility of the resin grains in the case of increasing the development speed.